AIPS HELP file for SKYVE in 31DEC21
As of Fri Dec 3 2:56:53 2021
SKYVE: Transforms DSS image coordinate system and projection
INNAME Input name
INCLASS Input class
INSEQ 0.0 9999.0 Input sequence, 0 -> high
INDISK 0.0 9.0 Input disk, 0 -> any
OUTNAME Output name
OUTCLASS Output class
OUTSEQ -1.0 9999.0 Output sequence
0 -> highest unique
OUTDISK 0.0 9.0 Output disk
0 -> highest with room
IMSIZE 0.0 16384.0 Output image size
BPARM Output map parameters
1) coordinate system
1: equatorial (default)
2) epoch of mean coordinates
3) epoch prefix (default: J)
1: Julian (eg J2000.0)
2: Besselian (eg B1950.0)
3: Besselian without
E-terms (eg b1950.0)
4) projection (default: NCP)
1: SIN 7: AIT
2: TAN 8: STG
3: ARC 9: CAR
4: NCP 10: MOL
5: GLS 11: PAR
5) blanking control
0: "magic blanking"
CPARM Output axis specification
See HELP for important
information concerning the
usage of CPARM.
1-5): first axis
1: hour (or degree)
2: minute (or arcmin)
3: second (or arcsec)
4: reference pixel
5: coord increment (arcsec)
6-10): second axis similarly
Use: SKYVE will regrid a Digitized Sky Survey (DSS) image to a
coordinate frame and projection recognized by AIPS.
The DSS is based on photographic material obtained using
the UK Schmidt Telescope operated by the Royal Observatory
Edinburgh (RGO), with funding from the UK Science and
Engineering Research Council (SERC) until 1988 June, and
thereafter by the Anglo-Australian Observatory (AAO). The
DSS was produced by the Space Telescope Science Institute
(STScI) under U.S. Government grant NAG W-2166.
Digitized Sky Survey images may be extracted from the CD
set as FITS files by a program called 'getimage'. This is
supplied with the CD set (on CD #61). The FITS file
created by 'getimage' may be read into AIPS using IMLOD.
The DSS image coordinate system is encoded as a set of
plate solution coefficients in FITS header cards which
are not generally recognized by AIPS. IMLOD stores these
unrecognized header cards in the history file associated
with the AIPS image. These header cards are listed in
the EXPLAIN file.
SKYVE retrieves the plate solution parameters from the
history file and regrids the image into a coordinate
system recognized by AIPS.
Why would you want to do this?
a) So you can use AIPS to measure positions from the
optical image. Verification tests done on a
selection of about 70 quasars with VLBI positions
by Martin Anderson (ATNF/UWS) using MAXFIT show that
an rms accuracy of better than 0.5 arcsec is
b) So that you can overlay radio images on top of the
If you want to overlay images then be sure to extract an
optical image slightly larger than the radio image to
avoid edge effects when the optical image is regridded.
You must also exercise caution in changing the pixel
spacing in the optical image to be much greater than that
of the original DSS image (approximately 1.7 arcsec). If
you make the spacing too great then stars and other small
scale objects may be skipped over. If you already have
the radio image then:
a) If the pixel spacing in the radio image is much
less than 1.7 arcsec then you should regrid the
optical image to the same spacing as the radio image.
You should be able to do this directly with SKYVE
using the IMSIZE, BPARM, and CPARM adverbs to match
the coordinate system and projection of the radio
b) If the pixel spacing in the radio image is much
greater than 1.7 arcsec then you should regrid the
radio image to the same spacing as the optical image.
This may be done with REGRD or HGEOM.
If you don't already have the radio image then you should
synthesize it with a cell spacing to match the optical
Coordinate transformations between the IAU1976 and
Bessel-Newcomb systems are done with full precision
assuming zero proper motion, parallax, and recessional
velocity at J2000.0
Specifying a Julian epoch 'J' to SKYVE implies that the
output coordinates are referenced to the new IAU1976/FK5
Specifying a Besselian epoch to SKYVE implies that the
coordinates are referenced to the old Bessel-Newcomb/FK4
system. An epoch prefix of 'B' indicates the convention
that the coordinates include the effect of the E-terms,
whereas 'b' indicates that they have already been removed.
FK4 catalogue coordinates were not corrected for the
elliptic terms of aberration (E-terms) except for
positions within 10 degrees of the pole. Most earlier
catalogues did not correct for them.
The default behaviour here is to assume that the E-terms
are included in all Besselian coordinates (including near
the pole). This can be defeated if it is known that the
input coordinates have already been corrected, or if it is
required that the output coordinates not contain them.
See the EXPLAIN section for a brief description of the
SKYVE reports statistics of the pixel displacements
(output map pixel coordinate minus input map pixel
coordinate) for the regridding operation.
The mean and rms for pixel displacements in X and Y are
reported, and also the correlation coefficient. If all
SKYVE defaults are adopted - same image size, J2000.0
equatorial coordinates, same centre coordinates and pixel
increment - then the mean shift should be approximately
zero, the rms should be a few pixels, and the correlation
coefficient much less than unity.
However, these statistics do not directly account for a
net rotation of the image, and this is usually the main
systematic difference between the output and input maps.
INNAME......Input image name, standard defaults.
INCLASS.....Input image class, standard defaults.
INSEQ.......Input image sequence number, 0 -> highest.
INDISK......Input disk drive number, 0 -> any.
OUTNAME.....Output image name, standard defaults.
OUTCLASS....Output image class, standard defaults.
OUTSEQ......Output image sequence number, 0 -> highest unique
OUTDISK.....Output disk drive number, 0 -> highest with
IMSIZE......Output image size (pixels), maximum 16384.
Defaults to the input image size if negative or
BPARM.......Coordinate frame and projection of the output map
1) Coordinate frame
0: -> 1
1: equatorial (mean of epoch)
3: ecliptic (mean of epoch)
Anything else produces an error.
2) Epoch of mean equatorial or ecliptic
coordinates, e.g. 1950, 2000.
Defaults to 2000.0 if negative or zero.
3) Epoch prefix
1: "J" - Julian (as J2000.0)
2: "B" - Besselian (as B1950.0)
3: "b" - Besselian without E-terms (eg b1950.0)
Anything else defaults to "J".
4) Spherical projection (geometry)
0: -> 1
1: SIN, sine (orthographic)
2: TAN, tangent (gnomonic)
3: ARC, arc (zenithal equidistant)
4: NCP, north celestial pole tangent
5: STG, stereographic
----- all sky types ----------------
6: GLS, global sinusoid (Sanson-Flamsteed)
7: MER, Mercator
8: AIT, Hammer-Aitov
9: CAR, Plate Carree ("cartesion")
10: MOL, Molweide's
11: PAR, Parabolic (Craster)
These should have ref latitude = 0 or they will be
"oblique" which you probably do not want.
Anything else produces an error.
5) output blanking control
0: "magic" blanking
(less than or equal to 0.5 -> 0;
greather than 0.5 -> 1)
CPARM.......Output axis specification
1-5) Apply to the first axis
1-3) Specify the coordinate reference pixel.
IF THE VALUE SPECIFIED IS OUTSIDE THE RANGE
-24HR TO +24HR, OR -360 TO +360 DEGREES, THE
COORDINATES OF THE CENTRE OF THE INPUT MAP
WILL BE USED (transformed to the coordinate
system of the output map if necessary).
1: hour for equatorial, degree for the others
2: minute for equatorial, arcmin for the others
3: second for equatorial, arcsec for the others
4: Coordinate reference pixel. If zero, the
centre of the output map is assumed.
5: Coordinate increment (arcsec per pixel,
should be negative). If zero, the DSS plate
scale is assumed.
6-10) Apply to the second axis as for the first
except the range is -90 to +90 degrees, and
the coordinate increment (if non-zero)
should normally be positive.
THE ABSOLUTE VALUES OF CPARM(6:8) ARE USED
TO COMPUTE THE DECLINATION (LATITUDE); IF
ANY OR ALL OF CPARM(6:8) ARE NEGATIVE THE
DECLINATION (LATITUDE) IS NEGATED.
SKYVE: Transforms DSS image coordinate system and projection
Author and documenter: Mark Calabretta, ATNF
Related tasks: REGRD, GEOM, HGEOM, LGEOM, PGEOM, COMB
For each pixel in the output image:
1) Compute its sky coordinates.
2) Transform to sky coordinates on the input map -
a) remove E-terms (only when the output map is in equatorial
b) perform the spherical coordinate rotation specified by
three Euler angles.
3) Compute the pixel coordinates on the input map. This
requires iterative inversion of the DSS plate solution
4) Interpolate the pixel value from the nearest nine pixels -
a) quadratic interpolation in X for each of the three rows.
b) quadratic interpolation in Y of the result.
Parameters used for the transformation (step 2) are recorded
in the history file.
The E-terms are recomputed for every pixel (step 2a,
computation of the E-terms is not done iteratively).
1) In RA: (E1*COS(RA) + E2*SIN(RA))/COS(DEC)
2) In DEC: (E2*COS(RA) - E1*SIN(RA))*SIN(DEC) + E3*COS(DEC)
For a spherical coordinate rotation from SYSTEM1 to SYSTEM2:
1) PHI0: Longitude of the ascending node in SYSTEM1.
Of the two points of intersection of the equators of SYSTEM1
and SYSTEM2, the ascending node is the one where the equator
of SYSTEM2 crosses from south to north as viewed in SYSTEM1.
2) THETA: The angle between the poles of the two systems.
Positive for a positive rotation about the ascending node.
3) PHI: Longitude of the ascending node in SYSTEM2.
Blank pixels are fully accounted for in the sense that one
blank pixel in the input map produces only one blank pixel in
the output map. The basic criteria is that the output pixel
will be blank if and only if the pixel (P0) on the input map
nearest the position computed at step 3 above is blank.
If P0 is not blank and any of the eight pixels surrounding
it are, then the quadratic interpolation reduces to a linear or,
if necessary, a constant interpolation or extrapolation. In the
worst case where all of the neighbouring pixels are blank, the
"interpolated" value would be the value at P0.
DSS FITS header cards
CNPIX1, The DSS pixel coordinates of the bottom
and CNPIX2 left-hand corner of the bottom left-hand
pixel of the image extracted by
PLTSCALE Approximate plate scale, arcsec/mm.
XPIXELSZ, Plate pixel size in X and Y, micron.
PLTRAH, J2000.0 right ascension of the plate
and PLTRAM, centre (hours, minutes, and seconds).
PLTDECSN, J2000.0 declination of the plate centre
and PLTDECD, (sign, degrees, arcmin, and arcsec).
PPO3, Plate centre offsets, micron.
AMDX1, ... Plate solution coefficients for the xi
to AMDX13 standard plate coordinate.
AMDY1, ... Plate solution coefficients for the eta
to AMDY13 standard plate coordinate.